1. Field of the Invention
This invention relates to driveshafts for motor vehicles.
2. Disclosure Information
Driveshafts connecting a forwardly mounted engine-transmission unit with a rear differential usually have a cylindrically shaped tubular body with swaged ends sized to be connected to universal joints. The tubular body dimensions are dictated by the maximum torque and the maximum rotational speed encountered by the driveshaft for the particular motor vehicle. The minimum diameter of an automotive driveshaft is primarily dictated by its minimum critical speed requirement. The minimum critical speed is defined as the minimum speed that damages the driveshaft. Consequently, the minimum critical speed must be higher than the maximum encountered rotational speed.
The driveshaft's minimum critical speed is a function of its bending moment of inertia. The bending moment of inertia increases toward the center of the tubular body as the square of the distance from the two supported ends. As a consequence of an increasing bending moment, the flexural rigidity of the driveshaft (i.e. the rigidity of the driveshaft with respect to deflection in bending) decreases toward the longitudinal center of the tubular body.
Because the flexural rigidity decreases toward the midpoint of the tubular body the necessary minimum shaft diameter has traditionally been calculated at this midpoint position. However, a cylindrically shaped tubular body with a diameter based upon the requirements at the midpoint position is actually larger and heavier than necessary. Because the flexural rigidity increases from the midpoint toward the ends of the body the diameter of the tubular body can conversely decrease from the midpoint toward the thickest section adjacent the ends of the body and the thinnest section near the longitudinal midpoint of the body. The thickness of the wall varies along the length of the body in an inverse relation with respect to the varying ends thereof. What is needed is a variable diameter driveshaft that has its largest diameter near its midpoint and is tapered down toward its ends over a substantial length of the driveshaft to maximum the strength and flexural rigidity of the driveshaft compared to its weight.
One typical driveshaft is disclosed in U.S. Pat. No. 3,659,434 issued to Wolf on May 2, 1972. The Wolf patent discloses a driveshaft with swaged ends to receive conventional U-joint yokes. The driveshaft has a middle cylindrical section and a short tapered section connecting the swaged ends to the middle cylindrical section. The interior of the driveshaft is filled with polyurethane foam.